Which sentence is correct? “The costume that I’m wearing for Halloween will knock your socks off!” or “The costume which I’m wearing for Halloween will knock your socks off!”? How about, “My Halloween costume, which I found at The Party Starts Here, will knock your socks off!” versus “My Halloween costume that I found at The Party Starts Here will knock your socks off!”? If you chose the first example both times, you’re right!
When I taught English composition, my students often asked me when to use which and when to use that—for example in “the field that they usually play on is still a swamp”—and what punctuation to use (or not) with each of them. This is probably something you don’t think about very much, unless you’re a grammar nerd like me. But there is, in fact, a standard guideline for making this choice, and—although more a matter of style than a rule—knowing and following it will help you write with confidence.
Which, that, and who are all relative pronouns. They express a relationship between two parts of a sentence—say, the subject and a phrase or clause that modifies it—and refer back to that subject, whether it be a person or a thing. The basic guideline for choosing which one to use is that, if the phrase is essential to the sentence—also called a “restrictive” clause—then you should use that with no commas. In the example above, “that I’m wearing for Halloween” is essential to describing “the costume”; it would not make much sense to say, “The costume will knock your socks off.” Your listener would wonder, which costume? In the second set of examples, on the other hand, the information about where I found the costume is non-essential information. The sentence would make just as much sense if we left that clause out entirely and said, “My Halloween costume will knock your socks off.” This is why this non-restrictive clause uses which and is set off with commas.
Let’s look at a few more examples. “The building that houses the Rooftop skybar is called Piedmont Place.” This essential modifying phrase uses that and is integral to the sentence—no commas required. On the other hand, in “The Rooftop skybar, which boasts beautiful views, sits atop Piedmont Place,” the presence of the view is non-essential information; the main message of the sentence is the Rooftop’s location. Therefore, the phrase uses which and is set off with commas—punctuation which lets us know the phrase could be left out altogether. We would get lost in the sentence, “The Rooftop skybar which boasts beautiful views sits atop Piedmont Place”; this sounds like a run-on sentence. On the other hand, “The Rooftop skybar that boasts beautiful views sits atop Piedmont Place” makes it sound like there are other Rooftop skybars that don’t have beautiful views. In the sentence, “The house that she sold is in Western Ridge,” ‘that she sold’ is essential to identify which house we’re talking about. Whereas, in “The house, which has 2000 square feet and a big fenced yard, sold in only two months,” you could leave out the non-essential phrase about its size without affecting the main import of the sentence. It is really just a matter of common sense. Using this guideline to decide between which and that results in clearer meaning.
A similarly careless, and even more annoying, switching of relative pronouns occurs between that and who. The rule—or guideline—here is simple: use who for people, and that for things! Calling a person that—as in, “the children that came to the door for trick or treat were not disappointed”—dehumanizes the children. Wouldn’t it be more accurate—and kinder—to say “the children who came to the door for trick or treat were not disappointed”? John Le Carre made a wise choice when he titled his 1963 thriller The Spy Who Came in from the Cold. “The Spy That Came in from the Cold” makes less sense and sounds clumsy to boot. Using that objectifies the subject. So it is better to say, “The cyclist who won the race trained hard” than “the cyclist that won the race trained hard.” But it is fine to say “the bicycle that he rode was a Trek Madone.” How unmusical to say, “I know an old lady that swallowed a fly!” In an interesting case of this usage choice, consider the two common versions of a popular quotation that adorns many magnets and posters. “It is never too late to be what you might have been” is sometimes rendered as “It is never too late to be who you might have been.” In this adage, often attributed to George Eliot without proof (see the 2/14/12 New Yorker story by Rebecca Mead), the choice of relative pronoun affects the meaning. Using what implies that you might have chosen a different career, whereas using who suggests that you might have (and still can) become a better person—more moral, say, or more successful. This difference is subtle, but that’s what we love about language!
The only remaining question is how to refer to animals. Should we say “the groundhog that lives under my porch,” or “the groundhog who lives under my porch”? This is not a trivial question; Yale held a 2013 conference to discuss just this issue, called Personhood Beyond the Human (I don’t believe they reached a conclusive answer). I’m sure my many pet-owning friends would agree that who is the proper pronoun to use when referring to Fido or Fluffy. And writers seems to agree, with such book and movie titles as The Dog Who Saved Halloween and The Dog Who Knew Too Much. But I’ll let you decide.